For linked fonts, whether the embedded font should be subset or not. Subsetting strips unused characters from the font, which results in smaller documents - although it does prevent the font from serving any useful purpose outside the scope of the document
This attribute only applies if the embed attribute is set to true, and is currently only implemented for TrueType fonts. Generally you'll want to leave it at the default of "true"
Embedding a TrueType font
<link name="myfont" type="font" subtype="OpenType" src="times.ttf" embed="true" subset="false"/>